Do the heart valves open at the same time?

>Yes

No they do not.

Left ventricle contracts before the right. (From the correct anatomical position.)

That makes sense because the left ventricle has more work to do, systemic vs. pulmonary.

http://www.nhlbi.nih.gov/health/dci/Diseases/hhw/hhw_electrical.html

Well guess what. This website is not entirely correct. Sometimes things are interpreted half right. What you want to focus on here from the EKG is the QRS complex. This is the depolarization and repolarization of the ventricles (not as they explained the firing of the left, then right ventricles). In general, you will say that the atria contract together and then the ventricles contract together. When you start splitting hairs, you can justify tiny differences in milliseconds, etc. Also, keep in mind that the left ventricle provides a much stronger contraction since it is sending out the blood to the systemic circuit, while the right goes out to the pulmonary circuit.